This invention relates to the preparation of polycarbonates, and more particularly their preparation by oxidative carbonylation followed by solid state polymerization.
Solid state polymerization (SSP) as a method for preparing polycarbonates is disclosed, for example, in U.S. Pat. Nos. 4,948,871, 5,204,377 and 5,717,056. Use of this method is of increasing interest by reason of its effectiveness and environmental benefits. It is typically described as involving three steps, of which the first step is the formation of a precursor polycarbonate, often an oligomer, typically by a reaction such as melt polymerization (i.e., transesterification) of a dihydroxyaromatic compound such as 2,2-bis(4-hydroxyphenyl)propane (bisphenol A) with a diaryl carbonate such as diphenyl carbonate. The second step is enhancement of the crystallinity of the precursor polycarbonate, and the third step is the building of molecular weight by heating the enhanced crystallinity precursor polycarbonate to a temperature between its glass transition temperature and its melting temperature.
Other methods of polycarbonate oligomer preparation are known. They include the oxidative carbonylation (hereinafter sometimes simply "carbonylation" for brevity) of a dihydroxyaromatic compound; i.e., its reaction with carbon monoxide and oxygen in the presence of a compound of a Group VIII element with an atomic number of at least 44, preferably palladium.
The carbonylation of both mono- and dihydroxyaromatic compounds by this method is disclosed, for example, in U.S. Pat. Nos. 4,096,168, 4,096,169 and 4,201,721. Further developments, of particular applicability to monohydroxyaromatic compounds, are the use of co-catalysts which may include an inorganic co-catalyst which is a cobalt compound, especially a complex with a pentadentate ligand, as illustrated by the cobalt(II) salt of bis[3-(salicylalamino)-propyl]methylamine, said complex hereinafter being designated "CoSMDPT"; and an organic co-catalyst, most often a terpyridine such as 2,2':6',2"-terpyridine. Reference is made, for example, to U.S. Pat. Nos. 5,231,210 and 5,284,964.
A still further catalyst constituent which is advantageously present for carbonylation of hydroxyaromatic compounds is a bromide (preferably) or chloride source, most often quaternary ammonium, quaternary phosphonium or hexaalkylguanidinium salt such as tetra-n-butylammonium bromide or hexaethylguanidinium chloride or bromide. The aforementioned U.S. Pat. Nos. 5,231,210 and 5,284,964 disclose the use of quater nary ammonium and phosphonium halides, and the similar use of cuanidinium halides is disclosed, for example, in copending, commonly owned application Ser. No. 08/929,000. The disclosures of all of the aforementioned patents and application are incorporated by reference herein.
Polycarbonate oligomers prepared by carbonylation are characterized by hydroxy end groups. Such oligomers are not generally suitable as such for SSP since their molecular weights are, for t he most part, too low, as exemplified by intrinsic viscosities (IV, in chloroform at 25.degree. C.) below about 0.10 and glass transition temperatures (Tg) below 100.degree. C.
They also have other disadvantages. In the first place, an essentially stoichiometric proportion of the expensive palladium compound is usually required for their preparation. In the second place, oligomer production is often low even when a stoichiometric proportion of palladium is employed, whether calculated in terms of percent yield based on dihydroxyaromatic compound or on "turnover number", the number of moles of carbonate units formed per gram-atom of palladium. In the third place, excessive reaction times on the order of 15 hours are frequently necessary.
The conventional second step of the SSP process, crystallinity enhancement, is considered essential in accordance with the aforementioned prior art. As taught, for example, in the aforementioned U.S. Pat. No. 4,948,871, the crystallinity of the precursor polycarbonate should be in the range of about 5-55% as determined, for example, from powder X-ray diffraction patterns. If it is below 5%, the melting point of the precursor polycarbonate is so low that melting rather than SSP may occur. On the other hand, at crystallinity levels greater than 55% the rate of the molecular weight building step is too low to be practical.
Crystallinity enhancement may be performed by several methods. These include heat treatment, solvent or non-solvent treatment, contact with crystallization promoters and treatment with swelling agents. Each of these methods requires time input and/or treatment with extraneous chemicals which must be kept in inventory and stored. It would be desirable, therefore, to develop an overall polymerization method, including a final SSP step, in which the precursor polycarbonate is inherently of sufficient crystallinity to make a separate crystallinity enhancement step unnecessary.